Evaporative media systems, for example direct evaporative coolers, are frequently used in commercial and industrial HVAC systems, including applications for data centers and power plant turbine inlet cooling. Evaporative media systems consume less energy than conventional cooling equipment and are increasingly being used to supplement and occasionally replace conventional cooling equipment. In operation, evaporative media systems use the enthalpy of vaporization of water as a means to cool and humidify air. Typically, this is accomplished by flowing air directly through a media wetted with water. As air passes through the wetted media, water evaporates by taking energy from the air to vaporize the water. Accordingly, the air temperature exiting the wetted media is reduced and the humidity is increased while the energy or enthalpy of the exiting air remains the same as the entering air. This type of a process is often referred to as adiabatic cooling.
One aspect of evaporative media systems is that the concentration of the impurities in the water (e.g. non-evaporating solids) in the storage tank increases as the water is evaporated by the media. If this concentration gets too high, scale will form. In order to keep scaling from happening, some of the concentrated water will need to be removed (drained) from the unit. As the water is evaporated and drained, it will accordingly need to be replenished. The draining of the water needs to be done such that a given cycles of concentration (COC) is achieved to control the amount of scaling and/or to prevent excessive scaling, but also such that water is not wasted by draining excessive amounts.
Commercial and industrial evaporative coolers used in HVAC systems control the COC have been configured in a variety of ways. One way uses a constant bleed rate of water drained from the unit. The bleed rate must be adjusted to generally match the output of the unit, which varies with the incoming air conditions and cooling demand, thus changing the actual COC maintained. Generally, the bleed rate method either wastes water from high bleed off rates and resulting low COC or causes fouling from insufficient bleed off rates and excessively high COC.
Another way uses drain events that occur at user configurable time intervals. The actual COC of the water is not known and varies because of the varying conditions and cooling demands. Yet another approach uses a conductivity meter to sense water conductivity in the tank. When the water reaches a user configurable threshold a drain event occurs and the unit is refilled. This approach can provide better COC control than some other methods, but does require the use of a conductivity meter in the tank. Additionally, for proper set-up the conductivity of the make-up water should be known. However, maintaining a target COC based on conductivity introduces errors. For example, since minerals in water precipitate as the water concentrates, the conductivity may not increase at the same rate as the COC. Improvements in COC control approaches for wetted media systems are desired.